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EATONTOWN, N.J. — State investigators worked Saturday to determine what caused an explosion that leveled part of a pet store, injuring five workers and apparently killing an unknown number of birds, reptiles and other animals.

Authorities were looking into whether a gas pipeline ruptured by a work crew had been properly marked, but state Board of Public Utilities spokeswoman Gloria Montealegre said it was too early to say exactly what was the cause.

Much of the roof and floor of the Petco store in Eatontown collapsed into the basement Friday. Bits of shattered glass were strewn more than 50 yards from the store.

Two in critical conditionFour people remained hospitalized Saturday, two in critical condition, at Jersey Shore University Medical Center. The fifth was released after treatment.

The explosion occurred as employees were evacuating customers and animals after construction workers damaged an unmarked gas line outside the store and then smelled gas, said Shawn Underwood, spokesman for San Diego-based Petco Animal Supplies Inc.

Volunteers with the Monmouth County Society for the Prevention of Cruelty to Animals worked until midnight to rescue about 25 parrots and parakeets whose cages had fallen into unstable areas of the basement. Rescue workers built ramps into the basement to help with the effort. No more rescues were conducted Saturday.

The birds were sent to a Petco store in Old Bridge, said Ursula Goetz, director of the Monmouth County SPCA.

Many animals believed killedMore than 100 small animals, including guinea pigs, gerbils, white mice and rats also were rescued, and buckets of fish were taken to nearby Petco stores in Woodbridge and Edison. Many other animals were believed to have been killed.

A representative of Long Branch-based J.F. Kiely Construction Co. had contacted the state’s One Call hot line before digging as required by law, said John M. Kiely, the company’s secretary and treasurer.

The state’s One Call system notifies utilities of impending work so they can mark the location of pipelines. However, the gas line that was ruptured was never identified, Kiely told The Star-Ledger of Newark.

New Jersey Natural Gas officials did not immediately return calls seeking comment Saturday, but they told The Star-Ledger that an independent contractor, Utiliquest, had accessed utility records to mark the site.